1. Field of the Invention
The present invention relates to a voltage reference circuit.
2. Background Art
FIG. 6 is a circuit diagram illustrating a conventional voltage reference circuit.
The conventional voltage reference circuit includes PMOS transistors 101 to 103, NMOS transistors 201 to 204 and 301, an output terminal 401, a power supply terminal 501, an earth terminal 502, and a resistor 601. A threshold voltage (hereinafter referred to as Vtnl) of the NMOS transistor 301 is lower than a threshold voltage (hereinafter referred to as Vtnh) of the NMOS transistors 201 to 204. The PMOS transistors 102 and 103 constitute current mirror circuits with the PMOS transistor 101 to flow a drain terminal current of a desired ratio to a drain terminal current of the PMOS transistor 101. The NMOS transistor 204 constitutes a current mirror circuit with the NMOS transistor 203 to flow a drain terminal current of a desired ratio to a drain terminal current of the NMOS transistor 203.
Source terminals of the PMOS transistors 101 to 103 are connected to the power supply terminal. Gate terminals of the PMOS transistors 102 and 103 are connected to a gate terminal and a drain terminal of the PMOS transistor 101 and a drain terminal of the NMOS transistor 201. Gate terminals of the NMOS transistors 201 and 202 are connected to a drain terminal of the NMOS transistor 201 and a drain terminal of the PMOS transistor 102. A source terminal of the NMOS transistors 202 is connected to the earth terminal. One end of the resistor 601 is connected to a source terminal of the NMOS transistor 201, and the other end thereof is connected to the earth terminal. Gate terminals of the NMOS transistors 203, 204, and 301 are connected to drain terminals of the NMOS transistors 203 and the PMOS transistor 103. Source terminals of the NMOS transistors 203 and 204 are connected to the earth terminal. A drain terminal of the NMOS transistor 301 is connected to the power supply terminal. The output terminal 401 is connected to a drain terminal of the NMOS transistor 204 and a source terminal of the NMOS transistor 301.
Respective K values of the NMOS transistors 201 to 204 and 301 are K201, K202, K203, K204, and K301, and a resistance value of the resistor 601 is R601.
The PMOS transistors 101 and 102, the NMOS transistors 201 and 202, and the resistor 601 constitute a constant current circuit. For example, in a case where each transistor works in a saturation region, if K values of the PMOS transistors 101 and 102 are equal to each other, currents flowing in the PMOS transistors 101 and 102 are equal to each other, and their current value takes 0A or a certain constant current value (hereinafter referred to as IK). When a start circuit is provided so that the current will not be 0A, the PMOS transistors 101 and 102, the NMOS transistors 201 and 202, and the resistor 601 work as a constant current circuit. The constant current IK is represented by the following formula:
                                          I            K                    =                                    1                              R                601                2                                      ·                                          (                                                      1                                                                  K                        201                                                                              -                                      1                                                                  K                        202                                                                                            )                            2                                      ⁢                                  ⁢                              where            ⁢                                                  ⁢            K            ⁢                                                  ⁢            201                    >                      K            ⁢                                                  ⁢            202.                                              (        1        )            
The constant current IK is mirrored to the PMOS transistor 103, and a drain terminal current of the PMOS transistor 103 is mirrored to the NMOS transistor 204. For example, when all the transistors of FIG. 6 work in a saturation region, K values of the PMOS transistors 101 and 103 are equal to each other, and when K values of the NMOS transistors 203 and 204 are equal to each other, the constant current IK flows in the NMOS transistors 204 and 301. When respective gate-source voltages necessary for the NMOS transistors 204 and 301 to flow the constant current IK are assumed VGS204K and VGS301K, a voltage (hereinafter referred to as VrefK) of the output terminal 401 is represented by the following formula with the use of the formula (1):
                                                                        V                refK                            =                            ⁢                                                V                                      GS                    ⁢                                                                                  ⁢                    204                    ⁢                    K                                                  -                                  V                                      GS                    ⁢                                                                                  ⁢                    301                    ⁢                    K                                                                                                                          =                            ⁢                                                                                          I                      K                                                        ⁢                                      (                                                                  1                                                                                                            K                              204                                                                                ⁢                                                                                                                                                    -                                              1                                                                              K                            301                                                                                                                )                                                  +                                  (                                                            V                      tnh                                        -                                          V                      tnl                                                        )                                                                                                        =                            ⁢                                                                                          1                                              R                        601                                                              ·                                          (                                                                        1                                                                                    K                              201                                                                                                      -                                                  1                                                                                    K                              202                                                                                                                          )                                                        ⁢                                      (                                                                  1                                                                              K                            204                                                                                              -                                              1                                                                              K                            301                                                                                                                )                                                  +                                  (                                                            V                      tnh                                        -                                          V                      tnl                                                        )                                                                                        (        2        )                                          where          ⁢                                          ⁢          K          ⁢                                          ⁢          201                >                  K          ⁢                                          ⁢          202.                                                
As mentioned earlier, the voltage reference circuit of FIG. 6 is a circuit to output a reference voltage VrefK determined by Vtnl, Vtnh, K201, K202, K204, K301, and R601.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-148530